Add a couple forward declarations, so that board support routines don't have
to pre-include a bunch of header files they don't need just to use this one.
Change the define in the header to eliminate unnecessary data
when using LEGACY TX.
Call soc_info.soc_data->soc_clock_init() before at91_pmc_init_clock(), so
that the latter correctly fills in the clock data structures based on
proper hardware-specific shift and mask values from the soc_data structure.
Change the API for at91_pio_gpio_get() to return the entire masked set
of bits, not just a 0/1 indicating whether any of the masked bits are on.
This is compatible with the single in-tree caller of this function right now
(at91_vbus_poll() in dev/usb/controller/at91dci_atemelarm.c).